Venus (planet), one of the planets in the
solar system,
the second in distance from the sun. Except for the sun and the moon, Venus is
the brightest object in the sky. The planet is called the morning star when it
appears in the east at sunrise, and the evening star when it is in the west at
sunset. In ancient times the evening star was called Hesperus and the morning
star Phosphorus or Lucifer. Because of the distances of the orbits of Venus and
earth from the sun, Venus is never visible more than three hours before sunrise
or three hours after sunset.

When viewed through a telescope, the planet
exhibits phases like the moon. Full Venus appears the smallest because it is on
the far side of the sun from earth. Maximum brilliance (a stellar magnitude of
-4.4, or 15 times the brightest star) is seen in the crescent phase. The phases
and positions of Venus in the sky repeat with the synodic period of 1.6 years.
Transits across the face of the sun are rare, occurring in pairs at intervals of
a little more than a century. The next two will be in 2004 and 2012.

IIEXPLORATION

Venus's complete cloud cover and deep atmosphere
make it difficult to study from earth, and most knowledge of the planet has been
obtained through the use of space vehicles, particularly those carrying probes
that descend through the atmosphere. The first flyby was that of Mariner 2,
launched by the United States in 1962, followed by Mariner 5 in 1967 and Mariner
10 in 1974.

The Soviet Union has developed several entry
probes, some combined with flybys or orbiters: Venera 4 and 5 (1967), 6 (1969),
7 (1970), 8 (1972), 9 and 10 (1975), 11 and 12 (1978), 13 and 14 (1981), and 15
and 16 (1983); Vega 1 and 2, sent toward Halley's comet in 1984, also flew by
Venus and released descent capsules. Several of these probes successfully
reached the planet's surface. The U.S. sent two Pioneer Venus missions in 1978.
Pioneer Venus 2 sent four probes to the surface, while the remaining craft
explored the upper atmosphere. Pioneer Venus 1, an orbiter, continues to measure
the upper atmosphere. The Magellan probe, launched toward Venus in 1989, began
transmitting radar images of the planet in 1990.

IIIATMOSPHERE

The surface temperature on Venus is highly
uniform and is about 462° C (736 K/864° F); the surface pressure is 96 bars
(compared with 1 bar for earth); the atmosphere of the planet consists of nearly
all carbon dioxide (CO2). The cloud base is at 50 km (31 mi), and the
cloud particles are mostly concentrated sulfuric acid. The planet has no
detectable magnetic field.

That 97 percent of Venus's atmosphere is CO2
is not as strange as it might seem; in fact, the crust of earth contains almost
as much in the form of limestone. About 3 percent of the Venusian atmosphere is
nitrogen gas (N2). By contrast, 78 percent of earth's atmosphere is
nitrogen. Water and water vapor are extremely rare on Venus. Many scientists
argue that Venus, being closer to the sun, was subjected to a so-called runaway
greenhouse effect, which caused any oceans to evaporate into the atmosphere. The
hydrogen atoms of the water molecules could have been lost to space and the
oxygen atoms to the crust. Another possibility is that Venus had very little
water to begin with.

The sulfuric acid of the clouds also has its
analogue on earth in a very thin haze in the stratosphere. On earth, sulfuric
acid is carried down in the rain and reacts with surface materials; indeed, this
so-called acid rain is damaging parts of the environment. On Venus the acid
evaporates at the cloud base and can only remain in the atmosphere. The upper
parts of the clouds, visible from earth and from Pioneer Venus 1, extend
as haze 70 to 80 km (44 to 50 mi) above the planet surface. The clouds contain a
pale yellow impurity, better detected at near-ultraviolet wavelengths.
Variations in the sulfur dioxide content of the atmosphere may indicate active
volcanism on the planet.

Certain cloud patterns and weather features can
be discerned in the cloud tops that give some information about wind motion in
the atmosphere. The upper-level winds circle the planet at 360 km/h (225 mph).
These winds cover the planet completely, blowing at virtually every latitude
from equator to pole. Tracking the motions of descending probes has shown that,
despite the scale of these high-speed, upper-level winds, well more than half of
Venus's tremendously dense atmosphere, near the planet's surface, is almost
stagnant. From the surface up to 10 km (6 mi) altitude, wind speeds are only
about 3 to 18 km/h (2 to 11 mph). The high-speed winds probably result from the
transfer of momentum from Venus's slow-moving, massive lower atmosphere to
higher altitudes where the atmosphere is less massive, so that the same momentum
results in a much higher velocity.

The upper atmosphere and ionosphere have been
studied in great detail by Pioneer Venus 1, which passes through them
once each day. On earth this region is very hot; on Venus it is not, even though
Venus is closer to the sun. Surprisingly, the night side of Venus is extremely
cold. (Day-side temperatures are 40° C/104° F, compared to night-side
temperatures of -170° C/-274° F.) Scientists suspect that strong winds blow from
the day side toward the near vacuum that is caused by the low temperatures on
the night side. Such winds would carry along light gases, such as hydrogen and
helium, which are concentrated in a night-side "bulge."

On earth the ionosphere is
isolated from the solar wind by the magnetosphere. Venus lacks a magnetic field
of its own, but the solar wind seems to generate an induced magnetosphere,
probably by a dynamo action involving its own magnetic field.

IVSURFACE FEATURES

Venus rotates very slowly on its axis, and the
direction is retrograde (opposite to that of earth). Curiously, Venus's synodic
year is almost exactly five solar days long, with the result that the same side
always faces earth when the two planets are closest. At such times, the side
facing earth can be viewed and mapped by earth-based radio telescopes.

In contrast to the very large antenna needed for
earth-based radar mapping of Venus, a modest instrument of Pioneer Venus 1
was able to conduct a nearly global survey. Combined with data from the Soviet
probes and from earth-based radar, the survey shows that the surface of Venus is
primarily a rolling plain interrupted by two continent-sized highland areas,
which have been named Ishtar Terra and Aphrodite Terra after two manifestations
of the goddess Venus. Aphrodite Terra, although not as high as Ishtar Terra,
extends nearly halfway around the equatorial region; it occupies the planet's
far side as viewed from earth at closest approach.

The more powerful radar aboard the Magellan
spacecraft has revealed huge active volcanoes, large solidified lava flows, and
a vast array of meteorite craters. The largest impact crater yet observed is
almost 160 km (100 mi) across—the smallest about 5 km (3mi). Although the
probe's radar could resolve even smaller craters, if any were present, Venus's
dense atmosphere apparently prevents smaller meteorites from impacting the
surface of the planet.

The global survey and other probes have also
revealed evidence that a great deal of tectonic activity has taken place on
Venus, at least in the past. Such evidence includes ridges, canyons, a
trough like depression that extends across 1400 km (870 mi) of the surface, and a
gigantic volcanic cone whose base is more than 700 km (435 mi) wide. The Soviet
probes have sent back photographs of the areas in which they set down, and have
also measured the natural radioactivity of the rocks. The radioactivity
resembles that of granite and strongly suggests that the material of Venus is
differentiated, or chemically separated, by volcanic activity. Angular rocks
that are visible in the Soviet pictures also suggest the existence of geologic
activity that would counteract the forces of erosion.